The four-stroke engine has achieved unparalleled acceptance including nearly universal use in internal combustion powered motor vehicles. The four strokes of such engines correspond to two full revolutions of a crankshaft and two reciprocal motion cycles of a piston in its cylinder, i.e., two downstrokes and two upstrokes. During the first downstroke, or intake stroke, a mixture of air and fuel or "charge" is drawn into the combustion chamber of the cylinder. The piston then returns towards the top of the cylinder to compress the charge during the compression stroke. The charge is then ignited, by a spark in non-diesel engines, and expands pushing the piston downward during the power stroke. In diesel engines, the charge ignites due to its own heat incident to compression. In the final or exhaust stroke, the piston rises to expel the combustion product.
In current motor vehicles, operation of the four stroke engine is controlled by reciprocating poppet valve systems. Such systems include at least one intake valve and one exhaust valve per cylinder. Each valve includes a valve head on the end of a valve rod that extends through an opening or valve guide, usually at the top end of the cylinder. A valve seat is formed at the opening. The valve is opened by lifting the valve head off of the seat into the combustion chamber, and is closed by returning the head to its seat. This motion is actuated by a cam rotating on a cam shaft. The cam is a disk-like element that contacts a bearing surface along its perimeter. The bearing surface is pressed towards the cam by a biasing spring. Most of the perimeter of the cam is circular, but one portion of the cam forms a lobe giving the cam a somewhat oblong shape. As the cam rotates, the lobe pushes the bearing surface outwardly during a portion of each revolution and then the bearing surface moves inwardly as the lobe passes. In the case of overhead camshafts, this motion is directly transmitted to the valve shaft to open and close the valve. In other cases, push rods, rocker arms and the like are employed to link the valve shaft to the bearing surface and cam.
Ideally, the intake valve would be open only during the intake stroke. That is, the intake valve would ideally open at top dead center of the piston cycle, remain open during the downward intake stroke, close at bottom dead center and remain closed until the beginning of the succeeding intake stroke. Similarly, the exhaust valve would ideally be open only during the exhaust stroke, i.e., from bottom dead center to top dead center. Both valves would ideally be closed throughout the compression and power strokes.
In reality, poppet valves do not work this way. Rather, the intake poppet valve typically opens during the exhaust stroke and remains open through the intake stroke and part of the compression stroke before closing. The exhaust poppet valve typically opens during the power stroke and remains open through the exhaust stroke and part of the intake stroke before closing.
The actual operation of the poppet valves as described above results in a number of anomalies. First, both valves are open at once during a portion of the cycle, i.e., during the last part of the exhaust stroke and the first part of the intake stroke. During this period, unused fuel passes from the intake port to the exhaust port resulting in waste and increased emissions. Second, the exhaust valve is open during the last part of the power stroke. As a result, the expanding charge, which is the payoff for the whole cycle, is allowed to blow out the exhaust rather than contributing to engine power. Third, the intake valve is open during the first part of the compression stroke resulting in lost compression and lost power. Finally, the exhaust valve is open during the first part of the intake stroke such that exhaust is sucked back into the combustion chamber together with the fresh charge. The net effect of these and other losses is that internal combustion engines under poppet valve operation typically achieve only about 40% or less of their theoretical work potential.
The anomalies noted above have been not only tolerated but considered essential by automotive engineers. Such engineers have noted, for example, that blowing unused and cool charge out the exhaust port helps to cool the exhaust valve so that higher compression ratios (e.g., 6:1) can be achieved without premature ignition or "dieseling." Mixing of hot exhaust gases with fresh charge during the intake is thought to result in improved atomization of the fuel and combustion. Moreover, practical limitations relating to the desired smooth profile of the cam and improved biasing spring wear prevent realization of ideal power cycle operation.
Even apart from timing considerations, conventional poppet valve systems are not very good aspirators. Ultimately, the potential power of an engine is limited by the amount of charge that the engine can throughput, and efficiency is limited by how completely fuel can be combusted and used. Both of these considerations require that engines process large amounts of air. Unfortunately, as poppet valves begin to open, the valve heads continue to obstruct flow thereby limiting the rate at which air cain be processed. The size of the circular intake and exhaust ports also limits air processing rates. Moreover, operating limitations typically require the fuel/air mixture to be maintained at about 14% fuel or richer, further limiting air processing.
Numerous attempts have been made to improve upon poppet valves including various systems that are generally referred to as rotary valves. Generally, though, these systems have not matured into practical designs. In many cases, rotary valves have failed to seal adequately under operating conditions resulting in poor compression, have worn quickly, have allowed lubricant to leak into the combustion chamber and/or have been unreasonably costly to produce. It is apparent that no fully satisfactory alternative to poppet valves has gained widespread acceptance in the industry.